This invention relates generally to phased array antennas and, more particularly, to phased array systems using ring slot radiator elements. Phased array antenna systems provide a convenient technique for steering antenna beams electrically. Each phased array system consists of a relatively large number of antenna elements that are separately fed with a radio-frequency (RF) signal to be transmitted. By controlling the relative phase of the RF signal in the separate antenna elements of the array, one can effectively steer a beam emanating from the array. If the array is two-dimensional, the beam may be steered about two axes. It will be understood, of course, that although such antennas are often described in terms pertaining to a transmitting antenna, the same principles also apply to steering a receiving antenna.
Although such antenna systems are well known, in radar and communications systems they have typically employed conventional radiator elements, such as horn antennas, helical antennas, or open-ended waveguide elements. These conventional radiator elements are prohibitively large in size and weight, and are relatively costly to manufacture, especially for operation at millimeter wave frequencies (30–300 GHz). There is a requirement in some applications for phased array antenna systems that have very closely spaced radiator elements, to provide fast scanning of pencil beams over a large search or coverage volume without forming a grating lobe. A grating lobe is an unwanted lobe in the antenna radiation pattern, caused by steering the beam too far in relation to the element spacing.
Use of ring slot radiator elements in phased array systems has been proposed for low frequency applications. For example, U.S. Pat. No. 5,539,415, issued in the name of Phillip L. Metzen et al., discloses an antenna system with an array of ring slot radiators. The same system is disclosed in a paper by Phillip L. Metzen et al., entitled “The Globalstar cellular satellite system,” IEEE Trans. Vol AP-46, no. 6, June 1998, pp. 935–942. The antenna array and associated feed probe structure disclosed in these publications is designed for operation in the L-band (1.61 GHz to 1.6265 GHz) and provides a very narrow (1%) bandwidth. Unfortunately, antenna systems of the type disclosed by Metzen et al. do not work at millimeter-wave frequencies, such as 35 GHz or higher. Moreover, the narrow 1% bandwidth is so narrow as to render the design very sensitive to manufacture, resulting in high production costs.
More specifically, one important reason that prior designs worked well at lower frequencies but not at millimeter-wave frequencies has to do with the difficulty of impedance matching a coaxial feed to a strip line mode for coupling to a ring slot radiator. At low frequencies, the thickness of a substrate on which the antenna array is formed is electrically quite thin (less than 2% of the operating wavelength). The feed probe, therefore, exhibits a negligibly small self-reactance, and transition from coaxial mode to the strip line mode requires little or no impedance matching. At millimeter-wave frequencies, a substrate of the same physical thickness has a significantly increased electrical thickness (about 12% of the operating wavelength). The self-reactance of the feed probe is, therefore, much larger, causing a serious impedance mismatch problem in the transition from coaxial mode to strip line mode.
Therefore, there is still a need for an antenna system using an array of ring slot radiators that can be operated at millimeter-wave frequencies, and preferably at a greater bandwidth. The present invention satisfies this need.